The present invention pertains to the aircraft control art and, more particularly, to an improved system for producing an aircraft stall warning signal.
The low speed aerodynamic performance of aircraft is typically based upon stall speeds determined from stall tests which are conducted with the aircraft's engines set at idle power. The idle power test is done because this condition provides the highest, and thus most conservative, stall speed. The idle power stall speeds are then used as the basis for setting the normal operational speeds of the aircraft at all power settings. The minimum usable speed within the normal flight envelope of the aircraft is the stall warning speed. By regulation, the minimum flight envelope speed for an aircraft must be at least 7% above the demonstrated stall speed.
The pilot of the aircraft is typically warned that the craft is approaching a stall condition by "stick shaker" action on his column. In modern commercial aircraft, the stick shaker condition is triggered by the angle of attack of the aircraft, as determined by an angle of attack vane, approaching the stall warning speed level. The pilot is trained to react to the stall warning by pushing forward on the control column to regain speed and thus avoid inadvertent aircraft stall.
A significant reduction in stall speed is encountered as the aircraft's engine power is increased. This is primarily due to the additional lift provided by the component of thrust acting in the lift direction on the aircraft. This effect is most pronounced for aircraft configurations having a high thrust-to-weight ratio. At landing flap settings, where thrust coefficients are high due to the low airspeeds attainable, full power stall speeds may be 10-15 knots lower than the idle power demonstrated stall speeds. This effect is illustrated in FIG. 1 which is a graph illustrating the relationship of aircraft lift coefficient C.sub.L to the aircraft angle of attack, .alpha.. Graph 12 plots the lift coefficient C.sub.L with the engine set at idle thrust. The stall condition is indicated at the peak of graph 12. To prevent the aircraft from actually flying to a stall condition, the stall warning, .alpha..sub.stall warning, is established at a lower angle of attack, as indicated.
Graph 14 illustrates the aircraft's coefficient of lift under maximum engine thrust. If the same stall warning angle of attack is used under maximum thrust conditions, it is apparent that the stall warning will be given at warning speeds which are lower than the idle power demonstrated stall speed. As such, undesirable excursions outside of the aircraft's usable speed envelope are possible since no advance warning is provided.
There is a need in the aircraft control art, therefore, to provide a stall warning system which produces an advance stall warning signal that is essentially independent of aircraft engine thrust level.